<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <script src="../js/jquery-1.11.3.js"></script>
</head>
<body>

<div>
    <li>aa</li>
</div>
<p>
<li>bbb</li>
</p>
<script>
    var a = 'div';
    var b = 'p';
    var c = 'li'
    //    $(a).css('color','red');
    //    $('div li').css('color','red');
    //    $(a + ' ' +c).css('color','red');
    $('div > li').css('color', 'red');
    $(a + ' > ' + c).css('color', 'red');
</script>
</body>
</html>